THE GENUS COLOBOCENTROTUS. 27 
side is found a set of three or four layers of yoke-shaped spicules (Pls. 16, fig. 
6; 45, fig. 8). In P. pedifera the yoke-shaped spicules are nearly straight 
(Pl. 16, fig. 8), with a row of small perforations on the under side of the 
spicule. In P. atrata the yoke-shaped spicule is well arched (PI. 16, fig. 12) 
with but few larger perforations. 
These perforations frequently pass into more or less prominent teeth (PI. 
45, figs. 4, 6). The yoke-shaped spicules of Colob. Stimpsoni (Pl. 45, figs. 
3-6) and Mertensii (Pl. 45, figs. 8,9) have fewer perforations than those of 
P. pedifera, but more than those of P. atrata. The position of the layers 
of the yoke-shaped spicules with reference to the basal disks is well seen in 
Pl. 16, figs. 6, 9, 10, and Pl. 45, fig. 8. Fig. 14, Pl. 16, and fig. 10, Pl. 45, 
show the atrophied tip of an abactinal pedicel. No spicules were found in 
the tubes of the pedicels. 
The changes due to growth in the appearance of the basal disk are shown 
in figs. 11 and 13, Pl. 16; the former is the basal plate of the pedicel of a de- 
nuded specimen of P. atrata measuring 8 mm. in diameter, the other of a 
pedicel from a specimen measuring 45 mm. with spines. 
The basal disks of the other species were taken from specimens varying 
between 70 and 88 mm. in diameter including the spines; P. pedifera, 82 mm. ; 
Colob. Mertensii, 70 mm.; Colob. Stimpsoni, 88 mm. 
THE PEDICELLARLEZ OF COLOBOCENTROTUS AND PODOPHORA. 
Plates 3°; 4-6; 17-19; 29; 30. 
The four kinds of pedicellariz of Podophora pedifera are figured on 
Pl. 3%, figs. 7-4. The rods vary but little in length, but the length of the 
necks differs greatly. In the tridentate pedicellariz (Pl. 3*, fig. 1) the neck 
is nearly twice as long as the rod; in the ophicephalous pedicellarie (Pl. 3°, 
fig. 2) it is but slightly longer than the rod. 
The heads of the triphyllous and globiferous pedicellariz are small (Pl. 3°, 
figs. 3, 4), that of the triphyllous pedicellariz being scarcely wider than the 
neck; this is about twice the length of the rod (PI. 3, fig. 3), while the neck 
of the globiferous pedicellarize is reduced to a minimum, the valves seeming 
to rest upon the head of the rod (PI. 3°, fig. 4). 
The heads of the tridentate pedicellarie of Podophora atrata seem 
to be somewhat more elongated and pointed (Pl. 3%, fig. 5) than those of 
P. pedifera, and have a much shorter neck. The ophicephalous pedicellarize 
